There’s a good chance you’ve been reading over the last few pages and thinking something along the lines of “That code is all well and good, but how do I make the PayPal callbacks work in my development environment so I can see if it even works?”
The problem, as I alluded to earlier, is that we need to send PayPal a URL that PayPal can use to tell our site that the user has authenticated and approved the payment. If you are developing the site locally, then you probably are testing it via something like http://localhost:3000, which is not a URL you can pass to PayPal. You need something that’s available on the open network that PayPal can see.
Generically, the solution to this problem is called “tunneling,” ...